1. Field of the Invention
The present invention relates generally to prosthetic devices and, more specifically, to a new heart valve prosthetic and bearing assembly therefor.
2. Description of Related Art
Prosthetic heart valves are mechanical valve devices that are implanted in a patient in order to replace a damaged or diseased natural tissue valve. In general, the known heart valve prostheses are comprised of an annular support member and at least one valving member operatively supported by the annular support member in order to provide a one-way flow of blood therethrough. The annular support member generally carries a suture ring on its exterior for attachment to the existing heart tissue.
As might be imagined, the replacement of a patient's heart valves with prosthetic heart valves places heavy demands, not only on the patient and the surgeon, but also on the designer of the prosthetic heart valves themselves. Designers are primarily concerned with patient compatibility and durability.
With regard to compatibility, heart valve prostheses must have exceptional hemodynamic (blood flow) characteristics that are comparable to that of a healthy, natural valve. Specific hemodynamic criteria might include minimization of pressure gradients on either side of an open valve; minimization or prevention of hemolysis, the destruction of red blood cells; minimization of stenosis, a narrowing of the valve opening that results in decreased blood flow; minimization of regurgitation or backward flow of blood through the valve; and minimization of turbulence that might provide resistance to smooth laminar blood flow.
Hemolysis, the destruction of red blood cells, is typically caused by crushing the red blood cells between parts of the valve during the opening and closing operation, and sometimes caused by shearing forces exerted on the cells as they rapidly pass by rough valve components such as worn parts or cloth. The crushed red cells that result from hemolysis may result in the formation of emboli or thromboli. Emboli are abnormal particles that circulate in the blood, whereas thromboli are abnormal particles that form a blood clot at a fixed position within a blood vessel. The minimization of hemolysis is paramount because the thrombus may break off, move through the patient's circulatory system and occlude a blood vessel, and thereby cause serious damage to organs, stroke, paralysis, and even death. If the thrombus resides near the prosthetic heart valve, it may result in the patient's death by growing large enough to cause valve thrombosis (i.e., forcing the valves to remain either open or closed). Valve thrombosis is frequently a sudden and catastrophic event.
Prosthetic heart valves, constituting foreign matter to the patient's body, tend to encourage thrombus formation. Because of this tendency, it is important to construct valve components from smooth and chemically inert materials. Moreover, the valve design must nearly eliminate crushing contact between moving valve parts.
Durability is as equally paramount as good hemodynamic characteristics, because an implanted heart valve prosthetic may open and close some 50,000,000 times each year. Durability is clearly parameter where such heavy demands are made on a device in which the patient's life lies in balance. Once a heart valve prosthetic has been implanted in a patient, it is unlikely that the heart valve will ever be replaced or repaired. In general, open heart surgery to replace or repair an existing heart valve prosthetic is riskier than leaving it in and living with the odds that it could fail.
Durability might be defined as a prosthetic heart valve's ability to withstand physical and chemical stresses produced within the heart. The goal of the designer is to avoid the necessity of reoperation and the possibility of gradual or catastrophic heart valve failure. In general, valves must be able to withstand large and rapid pressure changes, mechanical stress created by continuous opening and closing, and resistance to gradual change in physical and/or structural properties that may produce deterioration. Durability is affected by the material selected and the valve design. The best patient-compatible materials are thromboresistant and have a very low density. Unfortunately, such materials typically have very low resistance to mechanical stresses and exhibit high frictional characteristics.
DELRIN, the trade name for one manufacturer's polymer, is a good thromboresistant material, but has poor frictional and wear characteristics. Carbon/pyrolite, like DELRIN, is also a good patient-compatible material, but may be subject to fatigue damage at points of movement. At the other end of the spectrum, stainless steel alloys have good physical wear and frictional properties, but their high density and weight result in a prosthetic heart valve that requires an excessive amount of energy to open and close.
The known heart valve prostheses may be subdivided into a finite number of general valve types, all offering various advantages and disadvantages.
The known heart valves prostheses include: tissue valves; free-floating disc valves (FIG. 7A); ball and cage valves (FIG. 7B); pivoting disc valves (FIG. 7C); bi-leaflet valves (FIG. 7D); and the Bjork-Shiley valve (FIG. 7E). The free-floating disc valve and the ball and cage valve are generally considered undesirable because of the pressure gradients that develop on opposite sides of the open valve, and because the valve member (the disc or the ball) obstruct the blood flow and severely diminish the hemodynamics of the valves. The Bjork-Shiley valve was introduced in the late 1970s but was recalled because the two metal struts that control the movement of the disc were prone to stress and potential catastrophic breakage.
The present inventor is of the opinion that the pivoting disk and/or bi-leaflet disc valves, or variations thereof, are the best known choice of valve design for making a compromise between patient compatibility, good hemodynamics, and excellent durability. In general, pivoting leaflet valves are comprised of an annular support member, at least one valving member, and bearing means on opposite sides of the valving members to allow rotation of the valving members relative to the annular support member. In general, such bearing means is comprised of an axle member that protrudes outwardly from the valving member and a corresponding concave support surface that extends inwardly into the annular support member or, conversely, a valving member that extends inwardly from the annular support member and a corresponding convex support surface that extends inwardly into the valving member.
As might be imagined, the weak link in the durability characteristics of pivoting leaflet valves is the junction between the protruding axle member and the corresponding concave support surface. The axle member and the supporting concave surface physically contact one another, and because of continuous opening and closing, are subject to repetitive stress and frictional forces. For durability purposes, this application requires materials, such as stainless steel, having good frictional and wear characteristics. However, as mentioned above, the valve material must also be patient-compatible. In addition to being durable, it must have good thromboresistant characteristics and must not be so dense that excessive energy must be expended in opening and closing the valve. The problem is that there is no known material that possesses both patient-compatible characteristics and desirable physical characteristics.
The known pivoting leaflet prosthetic heart valves are typically comprised of carbon/pyrolite components. Carbon/pyrolite is generally considered the best compromise material for today's technology because it is relatively light, wear resistant, exhibits good frictional characteristics, and is thromboresistant. Unfortunately, carbon/pyrolite is brittle because it is manufactured by being externally hardened under high heat. Carbon/pyrolite is therefore subject to stress fractures at the rotational points of contact between the axle member and corresponding concave support surface.
In addition, the known pivoting leaflet heart valves having annular support rings and leaflets are assembled by compressing the annular support member on opposite sides and perpendicular to the rotational axis of the leaflets. The inward assertion of pressure at the described location deforms the annular support member and causes the concave support surface to expand outward just enough to allow the axle members of the leaflet to be inserted therebetween. Once the pressure is released, the concave support surfaces return to their original position to enclose the axle members of the pivoting leaflet.
The problem with the above method of installation, necessitated by the unitary carbon/pyrolite structure of the known prosthetic heart valve components, is that fatigue is introduced into the crystalline structure of the annular support member. This fatigue is very undesirable in prosthetic heart valves because it is possible that failure may later result therefrom. In addition, this type of prosthetic heart valve requires that an outer metallic ring be placed around the carbon/pyrolite annular support member in order to ensure that the annular support member does not fatally release the leaflets by expanding after implantation. Because the metallic retaining ring must be extremely rigid and relatively large, it must be detrimentally made of a material that typically is not thromboresistant. For example, the retaining ring is generally made from titanium. Moreover, the added metallic retaining ring necessarily reduces the Effective Orifice Area (EOA) of the valve because the outer dimensions of the total structure are limited by the anatomy of the patient.
U.S. Pat. No. 4,078,268, issued to Zinon on Mar. 14, 1978, discloses a bi-leaflet heart valve prosthetic, one embodiment of which includes a bearing insert threaded into the annular support member. The Zinon device, however, suffers from several infirmities. In particular, prior to reaching the threaded bearing plug, the axle member protruding from the leaflet must extend through and rotate within a cylindrical aperture provided in the annular support member. In addition to the frictional contact between the axle member and the wall of the cylindrical aperture, the Zinon device encourages thrombosis by crushing or tearing red blood cells in the space defined between these two rotating surfaces. Extreme clotting in this space may even prevent the valve from opening and closing altogether. Another problem with the Zinon device is that there is full surface contact, rather than point contact, between the bearing insert and the axle member, resulting in unnecessary friction and further exacerbating the possibility of thrombosis. Finally, the disclosed method of retaining the bearing insert in the annular support member is a threaded joint, which joint may loosen during use with disastrous life-threatening consequences.